1. Field of the Invention
The present invention relates to an image pick-up apparatus that generates a captured image, and a tracking method therefor.
2. Description of the Related Art
Conventionally, an image pick-up system that tracks a subject has been known as a monitoring apparatus using a digital camera or a surveillance camera.
For example, Japanese laid-open patent publication (Kokai) No. H06-054326 (JP06-054326A) discloses a system that calculates a motion vector of a subject based on pieces of image data and rotates a camera so as to track the motion of the subject. In order to track and shoot the subject effectively, this system rotates the camera at high speed when a zoom position is telephoto and rotates the camera at low speed when a zoom position is wide.
Japanese laid-open patent publication (Kokai) No. 2007-104200 (JP2007-104200A) discloses that controlling a WB (white balance) and an exposure within a tracking frame that is detected as a flesh color region optimizes performance of controls of a white balance and an exposure.
Thus, various applications, such as the subject tracking system and the apparatus to control a white balance and an exposure using the subject tracking function, have been proposed.
As these tracking methods, a tracking method using correlation between pieces of frame image data by which a motion vector is calculated, a subject tracking method performed based on image characteristic quantity, etc. are general.
However, the above-mentioned conventional image pick-up apparatuses have the following problems. That is, in the tracking method that uses correlation between pieces of image data as the above-mentioned tracking method, when a moving subject being different from a main subject crosses in front of the main subject, for example, the correlation between pieces of the image data of the main subject decreases, there is high possibility of tracking the moving subject that crossed in front of the main subject.
In the tracking method based on image characteristic quantity such as a tint, when a subject similar to a main subject exists in the surroundings of the main subject, it is difficult to continue tracking the main subject.
Also in the tracking method that is a combination of these methods, when the weak conditions for the respective methods are overlapped, it is difficult to continue tracking the main subject.
Japanese laid-open patent publication (Kokai) No. 2007-104200 (JP2007-104200A) discloses a condition for finishing subject tracking. That is, the subject is tracked by using data of an angular velocity sensor and AF (auto-focusing) data, and it is determined that the subject is lost when difference between characteristic quantity of the subject within a tracking region and that at the time of a tracking start becomes large.
However, even when a subject cannot be tracked temporarily, for example, when a car or the like crosses in front of the subject, or when a detected face as the subject turns sideway temporarily, there is a sufficient possibility of returning to the state that allows the tracking within a predetermined time.
In such a case, if a time during which the subject cannot be tracked is temporarily short, the system can be controlled without loosing the subject by fixing the tracking region temporarily and by restarting the tracking again when a subject that has high correlation with the original subject is detected again after the lapse of time. This enables to improve the ease-of-use of the apparatus.
Here, the image pick-up apparatus provided with such a subject tracking function has a function that generates an image on which a frame (a tracking frame) is superimposed on the tracking region and displays it on a monitoring device, in order that a user can check easily whether the subject tracking is succeeding or not.
When a main subject is lost, the tracking frame should be eliminated. However, if the tracking frame is immediately eliminated after loosing, the tracking frame returns immediately when the main subject is found immediately after that, which may serve as rather unseemly display.
Therefore, when the main subject is lost temporarily, the display of the tracking frame should be continued, but when the main subject is completely lost, the display of the tracking frame should be eliminated. However, the decision thereof is extremely difficult.